If you play you will understand some things it's good that you are questioning. I will answer what I know logically.
Boss: It's something bigger than you in your lvl it might be sleeping and you need to awake it and it has like crazy statistics from you (Health, M.Def, P.def, etc) that's why it's called Boss. Examples? (Baium, Galaxia, Typhoon, Hestia).
Grand Boss: (The same +25% boost in every stat)
Epic Boss: (The same +25% more stats from Grand Boss)
Endgame Boss ( The same +25% more stats from Epic Boss)
Epic Endgame Boss ( It might the most powerful I don't even battled any kind of this shit it must be powerful I believe 50% more stats of EndGame Boss).
Mage, Nuker, = The same it's just other name.
Enchanter: It's just another mage class don't bother is useless.
Buffer: Buffer you propably reffering for low servers because buffers are boosting you with different kind of buffs. Also, there are different kind of buffers. (For example one general buffer is prophet from race human).
Buffers are mages but they cannot hit or make damage it's like supporters.